WASHINGTON — Shortly before leaving the annual meeting of major world powers on Saturday, President Trump boasted that it had been “tremendously successful” and that on a scale of 0 to 10, “the relationship is a 10.”

Fewer than nine hours later, the relationship was plummeting toward a zero. With a petulant tweetstorm from Air Force One, Mr. Trump all but blew up the Group of 7 nations that the United States has led for more than four decades and essentially declared open political war on America’s closest neighbor.

Whatever hopes that other leaders had of pacifying Mr. Trump and papering over their widening differences on trade, security and the world order vanished in a flurry of harsh recriminations. Prime Minister Justin Trudeau of Canada rebuffed Mr. Trump’s positions in public comments, prompting Mr. Trump to refuse to sign the carefully crafted final communiqué.

The blowup left the United States alienated from its allies even more than it had been entering into the summit meeting and came as Mr. Trump flew to Singapore, earlier than originally scheduled, for a risky meeting with a nuclear-armed American enemy, Kim Jong-un, the repressive leader of North Korea.

“Trump is readier to give a pass to countries that pose a real threat to Western values and security than to America’s traditional allies,” said Peter Westmacott, a former British ambassador to Washington. “If there is a ‘method to the madness,’ to use the words of British Foreign Secretary Boris Johnson, it is currently well hidden.”

During his two days in Canada, Mr. Trump and his counterparts from the Group of 7 shook hands, mugged for the cameras and pretended they were friends. White House officials insisted the encounters were congenial. Negotiators struggled to come up with a compromise statement that all seven powers could agree to and, in a burst of relief, released it Saturday night.

But as the bruising aftermath made clear, the divide, for the moment at least, cannot be bridged by clever diplomacy and cordial talking points. Mr. Trump’s view of the world, and his country’s oldest and most important partners, is so infused by suspicion and grievance that he could not resist his pugilistic impulses even for a day.

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The conference of industrialized nations, known as the Group of 7, was the Group of 8 before Russia was expelled after annexing Crimea in 2014.Published OnJune 8, 2018CreditCreditTom Brenner/The New York Times

Mr. Trump’s unvarnished post-summit Twitter insults aimed at Mr. Trudeau — “weak & dishonest,” “false statements,” “meek and mild” — left the Canadian-American relationship at its most overtly hostile since, perhaps, the War of 1812. Indeed, Mr. Trump had already clashed with Canada before the summit meeting by reportedly accusing it of burning down the White House during that war (it was really the British).

Mr. Trump seems unbothered by the ostracism, making the case that America has allowed itself for too many years to be pushed around by foreign powers that took advantage of it. In some ways, he wears the criticism from abroad as a badge of honor, proof that he is representing the best interests of the United States. Supporters cheer his aggressive posture on the world stage.

Mr. Trump had already made clear before the Group of 7 summit that he had no intention of reconsidering his position on climate change or Iran. And he refused to ease steel and aluminum tariffs he just imposed on Europe and Canada, arguing that the allies have been dealing with the United States unfairly.

“There are disagreements,” Larry Kudlow, the director of Mr. Trump’s National Economic Council, conceded at one point during the summit meeting. “My view? We can get through this.”

No one got through it this weekend, however. Mr. Trump signaled his disdain for the Group of 7 meeting by arriving late and leaving early. During closed-door meetings on Friday, he went around the room, citing ways each of the other nations represented there had mistreated the United States in some fashion or another, according to a European official.

Just hours before Mr. Trump’s outburst, a senior administration official said the meetings had been less contentious than depicted and that the session with Mr. Trudeau in particular had been much better than anticipated. The official, who insisted on anonymity to describe closed-door discussions, predicted the United States would sign the final communiqué.

But Mr. Trump, while en route to Singapore, evidently watched or was briefed on a news conference Mr. Trudeau held afterward and was offended by the Canadian’s defense of his country’s trade policies.

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Russian soldiers in 2014 in the Crimea region of Ukraine. Mr. Trump has never expressed the outrage at Russia’s actions in Ukraine that European leaders feel.CreditSergey Ponomarev for The New York Times

Even before then, Mr. Trump had left himself the outlier in the annual session. By departing before the end, he skipped sessions about climate change, oceans and clean energy — ceding not only America’s leadership on those topics, but also its very seat at the table.

And with no warning on Friday, Mr. Trump called for Russia to be reinstated as a member without insisting on any of the conditions the West has demanded in terms of ending its intervention in Ukraine.

Mr. Trump pressed the point on Saturday, making clear that Russia’s annexation of Crimea should not stand in the way. “It’s been done a long time,” he said. “I would rather see Russia in the G-8, as opposed to the G-7. I would say that the G-8 is a more meaningful group than the G-7, absolutely.”

Mr. Trump was at odds not only with America’s allies but also with many in his own administration. Even as he was clashing with European leaders on Friday about a Russian return to the Group of 7, his director of national intelligence was giving a speech in France condemning Moscow’s malign behavior and warning against allowing it to divide the allies.

Speaking at a conference in Normandy, Dan Coats, the intelligence director, outlined a series of damaging actions by Russia in addition to its annexation of Crimea, including its efforts to interfere in American and European elections and the poisoning of a former Russian spy and his daughter living in Britain.

“These Russian actions are purposeful and premeditated and they represent an all-out assault by Vladimir Putin on the rule of law, Western ideals and democratic norms,” Mr. Coats said, according to prepared remarks. “His actions demonstrate that he seeks to sow divisions within and between those in the West who adhere to democratic norms.”

“The Russians are actively seeking to divide our alliance,” Mr. Coats added, “and we must not allow that to happen.”

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President Vladimir V. Putin of Russia on a visit to China on Friday.CreditAlexei Druzhinin/Sputnik, via Reuters

Some foreign policy experts said Mr. Trump had a point about bringing Russia back to the table with the Group of 7. Jeremy Shapiro, a former State Department official who has been critical of the president, said “every once in a while he gets it right.” The breakdown of the West’s relationship with Russia stems from the failure to recognize Russia’s legitimate status in the world, he said.

“The G-7, G-8, is a big symbol of that,” said Mr. Shapiro, now the research director at the European Council on Foreign Relations, based in London. “Letting them in was a big deal, and kicking them out was a big deal. We have to get past the notion that going to these meetings is some kind of reward.”

But Molly McKew, a foreign policy strategist who served as an adviser to the former Soviet republics of Georgia and Moldova in their struggles with Moscow, said the expulsion of Russia from the Group of 8 reflected “a sense of unity” in the West that has eroded since Mr. Trump’s election, encouraging some European nations to want to return to doing business with Russia.

“As Trump’s America is increasingly isolated and viewed as a rogue actor, the Kremlin is again preying upon ‘economic openness’ to buy silence for their crimes in Ukraine, Syria, and beyond, even as they attack our societies,” Ms. McKew said.

That may be true in the long run. But for now, Mr. Trump’s actions have united allies against the United States — not just because of policy but because of a personal style that has confounded and sometimes infuriated his counterparts, who never know quite what to expect.

“On a Monday, you could be his best friend, on Tuesday his worst enemy, and by Friday, you’re golf buddies again,” said Mark Dubowitz, the chief executive officer of the Foundation for Defense of Democracies.

The Group of 7 nations struggled to reach consensus for a joint statement that Mr. Trump could sign. The American delegation objected to including the standard phrase “rules-based international order” only to back down in the final talks. But the negotiations were for naught after Mr. Trump balked at signing.

“The G-7 should be our preferred venue to unify the free world to compete with and counter authoritarian kleptocracies,” said Damon Wilson, a former national security aide to President George W. Bush and now vice president of the Atlantic Council, which advocates trans-Atlantic cooperation. “Rather than prepare for that real battle, we’re distracted in a family dispute.”

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Lobbying for Russia, And Deepening a Rift With America’s Allies. Order Reprints | Today’s Paper | Subscribe